Garage Retaining Wall - Requirement for Structural Engineer or Not?

Hi Everyone,

I face a dilemma, and don't know who to believe or the implications. I am building an attached single skin 4"/100mm concrete block garage to the side of my house, with flat roof. It projects into sloping ground at the rear, and beyond the rear wall of the house.

The rear (end wall) will line up with and be tied in to an existing single skin 4" concrete block patio wall which has been in place 40-50 years, retaining the garden successfully, without any sign of bowing or collapse (in spite of having a 9" dia cherry tree tree growing against it). The ground above is gently sloping upwards and away, and the bank outside the garage-to-be is approx 110cm above finished floor level. The ground is stable topsoil over chalk at just below floor level (Salisbury Plain).

The outside, long wall will be against the bank from the corner (110cm) reducing to 80cm after 4800cm, where it meets my neighbour's concrete block 'shed' (done all the Party Wall stuff, no issues, and successfully removed identical, adjoining shed with disc cutter, mini-digger, etc). The long wall is continuous, not attached to 'shed' and set just inside boundary to allow for fascia and gutter Beyond the 'shed' the same wall (total 9250cm, with 3 intermediate piers) has no bank against it. The garage width (end wall) against the bank is 375cm. The second long wall returns to the house at 400cm (internally, garage continues with existing house wall).

Trouble is, BCO either wants engineer's design for retaining wall, or says he will accept secondary 'garden' wall outside garage build, with gap (as it's "not part of project" for inspection), thereby losing me internal dimension of garage. Builder says lay concrete blocks flat to above bank level (with appropriate membrane and drainage outside) then resort to normal courses above. Internally, I end up with step effect in wall across end and part of outside long wall, but better than loss of space caused by two walls with gap. Also, the corners at either side of the end wall provide strength in their own right, and the end wall could/will have a central pier depending on whether built with blocks flat or traditional course from floor upwards.

If I have no choice, what might an engineer's design cost, please? How can I reason with BCO as his alternative proposal not only loses space, but causes potential damp problem with build up of detritus in gap (BCO would not commit on size, and would accept anything!) ? Given the existing patio wall, I don't understand the BCO's logic. Any experienced or professionally qualified comments will be appreciated. Thank you.

 

Thank you for reply. Good point about stepping outside. Will be tight to render in available space, before back-filling, but certainly worth considering. Biggest concern is strength, and how to argue/prove reason with BCO.

 

Kiab, understood thank you. Because digging is over boundary for footings, I daren't take more as the neighbours can be difficult. On the end wall, no problem. Will wait to hear other replies.

 

Thank you both for reply, especially Stevie for responding to separate request. The chalk and sub-surface are good and firm. I recently dug out behind a pre-cast (panelled) concrete garage of mine (block of 4, 30 metres away off my property, but same terrain), then placed permeable membrane against walls before back filling with large clean stone (5 tonnes). That has done a great deal to stop creeping ingress along wall and floor plate. How much reasoning will BCO accept? When I asked on his first pre-build visit he pointed out the issue but wouldn't commit on how best to resolve it. The next time I see him is when he's called to sign off the footings, and I'm prepared to discuss it then. However, although he obviously knows the answers (I presume) he didn't seem willing to agree much, hence the suggestion of a secondary 'garden' wall or a structural engineer. Saving SE costs would be appreciated, and I can't find a simple, easy to find reference that gives me the common-sense answer I seek. I understand the equations about load, etc but it hardly seems relevant given the obvious lack of previous movement, the low ground height, and the short spans involved. Also, said BCO talked to me about distance for piers (drawing for planning permission was simplified and showed two on long wall) and had an idea of required distances, but rather than tell me the answer when he looked it up in the office, he instead sent me the reference (requires three). Same with roofing timbers/rafters to span 3.75m. He could have told me, but no, he sent me the TRADA document reference (which I'd already read and downloaded). I don't need to be spoon-fed but I'm a householder, and I presumed the BCO was there to help 'ordinary' people through the process.

 

Stevie, thank you, point taken and understood, especially about slow movement but there is no traffic, water-course or anything like that. I'm not being dismissive but this is a back garden to a cottage set in agricultural land (MOD) which will not be developed or changed. The garage is single skin, deliberately, so that it cannot easily be built over and turned into an upstairs extension (though the footings will be sufficient). I appreciate your input and am grateful to you. I'll have to see what the BCO says, even if it causes delay. In the meantime I'll see if I can find a local SE.

 

By way of an update.......... BCO provided an interesting solution himself, but not the one I expected or wanted really as it effectively caused us additional hassle and extra expense. His 'solution' was to build a separate 'garden wall' out of the same 4"/100mm standard 7Nm concrete blocks to the height of the garden ground level. Provided it was not tied to the garage it would not be regarded as part of his project inspection, and didn't fall into the realms of being a 'retaining wall' built to an engineered standard! He stated that the stand-off distance could be as little or as much as I wanted. I therefore elected to place the garden wall and garage wall c4" / 100mm apart, which allowed me to put a separate perforated drainage pipe covered with gravel between, with weep holes through the garden wall. The level of gravel was below the floor raft level, so there can never be a build up of water between the two walls. In order to stop a build up of detritus between the two walls I intend to place Celotex/Kingspan type foam cut from a 100mm board into the top between the walls (this was useful to stop mortar dropping in between when building above garden wall level) then finally 'bench' or 'haunch' it, as I have to render the visible wall at the end of project (stipulated, in my scenario, by the Planners). My builder is still bemused at the BCO's decision, and is convinced that others would have accepted the simple solution of building the lower part of the garage wall to ground level by constructing concrete blocks on the flat, thereby doubling everything up.

 

Got it in one! From the start, that was the way we intended to do it. Double thick below ground, then single skin, traditional stretcher courses above. Unfortunately, when asking (BCO and others), common sense didn't apply, even though it was a recognised solution agreed by several highly experienced third parties. Nothing more to say, after all, facts are facts and we all share different opinions.